The present invention relates to a glaucoma drainage implant which allows fluid to filter out from the eye. One in 30 Americans over the age of 40 has some form of glaucoma. All types of glaucoma are characterized by an excessive accumulation of aqueous humor in the anterior chamber of the eye, which results in increased intraocular pressure, visual field loss, optic disc cupping, and when left untreated, blindness. To date, there is no known cure for any type of glaucoma.
Initially, medical therapy, i.e., topical medicine, is used to increase outflow and/or decrease production of aqueous humor. However, approximately, 40-50% of patients with glaucoma eventually fail medical therapy, and become candidates for complex forms of filtration surgery. These procedures, including full-thickness sclerectomy, trabeculectomy, and drainage implants, involve the making of a channel from the anterior chamber to the subTenon's space. The aqueous pool that accumulates under Tenon's layer forms a bleb from which the fluid is absorbed by the surrounding tissues.
One of the main reasons for the 10-50% failure rate of filtration surgery is inadequate reduction of intraocular pressure due to bleb failure (shunt closure) over either the short-term (weeks to months) or long-term (years) postoperative course. After the immediate postoperative period, the inability to maintain reduced intraocular pressure is the single most common serious complication of the three principal forms of filtration surgery mentioned above. Eyes with failed filtration surgery usually have no observable filtration bleb or have a thick-walled encapsulated bleb that is nearly impermeable. The bleb fails due to excessive healing characterized by fibroblastic proliferation and subconjunctival fibrosis. In eyes with inflammation, neovascularization, or previous intraocular surgery, this process of scarring is especially excessive.
Wound healing is a postinflammatory process that represents the effort of the body to repair and to restore the integrity and function of damaged tissue. The wound healing in response to filtration surgery is complicated by the presence of an activating factor (the aqueous humor) and, in the case of drainage implants, the presence of the implant itself. Encapsulation by a fibrous capsule is the normal wound healing response of the host to an implant and to the associated local trauma caused by the implantation process. The extent of the development of the fibrous capsule adjacent to a prosthetic implant is related to both the size and surface characteristics of the implant as well as to the degree of chemical inertness.
For implants in dynamic situations, such as vascular grafts, tendons, and patches on moving organs (heart, stomach, striated muscles, intestines), being smooth and inert is a hindrance to long term anchoring. Integration between the implant and the host is necessary to prevent detachment or shifting of the implant. Sutures are often inadequate for this purpose because simple tacking sutures allow the implant to shift and bulge. Suturing the entire length and circumference of the implant introduces the possibility of leakage of the internal organ fluid such as blood and bile through the suture holes and impairment of the strength of the implant. On the outside of the eye, porosity of the implant that allows integration of the host tissue would be very important in stabilizing the micromovement of an implant in such a dynamic environment. Infiltration of tissue into the implant from the sclera would increase the apposition between the bottom surface of the implant and the upper surface of the sclera to an extent that neither sutures nor a fibrous capsule would be able to approximate.
Failed long-term drainage implants in human eyes have been shown to have dense fibrous capsules that ultimately cut off flow from the plate region. One reason for development of such a thick capsule around the implants is the chronic micromovement of the implant against the scleral surface, as discussed above. By designing a drainage device with porous cellular-attachment surfaces on the posterior plate surface and around the exterior surface of the tube, the device will be able to fix in place at the bleb site during the early healing period so that it will be sufficiently immobile as to offer little to no stimulus for further fibroblast activation.
The use of cellular attachment around or through the drainage device is a known means of improving the apposition between the device and the surrounding tissues. U.S. Pat. No. 5,338,291 to Speckman et al. uses texturing of the silicone surfaces of the drainage device plate to interrupt the formation of a dense fibrous capsule around the episcleral plate and to promote vascularization around said plate. The textured surface of Speckman's plate comprises a plurality of fingers extending generally uniformly and outwardly from the episcleral plate. The textured surfaces are used to cause an interruption of the fibrous capsule formation resulting in multi-planar collagen deposition. Additionally, U.S. Pat. No. 5,397,300 to Baerveldt et al. uses holes which extend through the plate to facilitate the formation of a tethered scar tissue bubble/bleb. It is intended that the scar tissue will grow through the hole or holes and pull the perimeter of the bubble/bleb towards the episcleral plate at the hole locations to tether the formation of the bleb through the plate to the scleral tissue.
However, the texturing system disclosed by Speckman has certain disadvantages. The cells and collagen of the fibrous capsule wrap around the perpendicular length of the projections but do not attach to them. Therefore the fibrous capsule response around the finger-like texturing only helps to immobilize the implant from side to side motion in the plane parallel to the sclera. There remains a small space between the texturing and the capsule which allows the implant to move perpendicular to the sclera in response to the dynamic movements of the eye, such as muscle movements to produce accommodation, and flexure of the sclera produced by blinking and intraocular pressure fluctuations. Thus the micromovement of the device is somewhat decreased but not eliminated with the texturing.
Also, Speckman teaches that the anterior surface of their device is textured in addition to the posterior surface to interrupt dense fibrous capsule formation and to promote vascularization around the plate. However, this will allow the fibrous capsule to form around the texturing points on the anterior surface as well as on the posterior surface. The area for the aqueous humor to flow over and spread (bleb space) is significantly decreased with the microtexturing and is almost completely eliminated when the fibrous capsule forms around the texturing.
The holes taught by Baerveldt have other types of disadvantages. The holes require that the cells grow through the thickness of the plate, generally one millimeter, isolating themselves from other cells. The circumferential size of the holes, particularly in view of the thickness of the plate and the smooth surface of the hole walls, present a very difficult surface area for the cells to try and attach to while growing. The overall effect of these parameters limit cellular ingrowth through the holes. Additionally, if sufficient holes are placed throughout the episcleral plate to facilitate the cellular ingrowth through them, then there would no longer be sufficient surface area on the anterior surface of the plate to disburse the aqueous humor flowing thereon, and the tethers of cellular tissue through the plate would act as physical barriers impeding the dispersion of the aqueous humor.